Power IGBTs are described, for example, in Stengl, Tihanyi: “Leistungs-MOS-FET-Praxis” [Power MOS-FET practice], Pflaum Verlag, Munich, 1992, pages 101-104 or in Baliga: “Power Semiconductor Devices”, PWS Publishing, 1995, pages 428-431.
FIG. 1 shows part of a cross section through a vertical power IGBT. This IGBT comprises a semiconductor body 100 having an emitter zone 11 which, in the component according to FIG. 1, is arranged in the region of a rear side of the semiconductor body 100. This emitter zone 11 is also referred to as a collector in the case of an IGBT. The emitter zone 11 is adjoined by a drift zone 12 which is complementarily doped with respect to the emitter zone 11. A cell array having a number of identically constructed transistor cells is present in the region of a front side of the semiconductor body 100 that is opposite the rear side. Each of these transistor cells comprises a source zone 15 and a body zone 14 which is arranged between the source zone 15 and the drift zone 12, the body zone 14 being complementarily doped with respect to the source zone 15 and the drift zone 12.
In order to control a conductive channel or an inversion channel in the body zone 14 between the drift zone 12 and the source zone 15, there is a gate electrode 16 which is arranged adjacent to the source zone 15 and the body zone 14 and is insulated with respect to the semiconductor body by means of a gate insulation layer 17. The body zones 14 are arranged at a distance from one another in the drift zone 12. The body zones 14, for example, have a rectangular or hexagonal cross section in a plane that runs perpendicular to the plane of the drawing illustrated in FIG. 1. The gate electrode 16 is in the form of a grid in this plane, as described, for example, in Stengl, Tihanyi, loc cit., page 33, and has recesses which are used by a terminal electrode 18 to contact-connect the source zones 15 and body zones 14 of the individual transistor cells and to thereby short-circuit these zones 14, 15. In this case, the gate electrode 16 is insulated with respect to this terminal electrode 18 by means of a further insulation layer 19.
The vertical power IGBT turns on when a positive voltage is applied between the emitter zone 11 and the terminal electrode 18, which is also referred to as a source electrode, and when a suitable drive potential for forming an inversion channel in the body zone 14 is applied to the gate electrode 16. When the IGBT is on, the drift zone 12 is flooded with p-type charge carriers or holes which, when the power IGBT is being switched off, must flow away, via the body zones 14, to the terminal electrode 18 which is at the lower potential. When switching off the component, it is necessary to ensure that a change in the gate potential for turning off the component takes place so slowly that a temporal change in the voltage, which is applied across the component, or a temporal change in the current, which flows through the component, does not exceed prescribed limiting values during the switching-off operation. These limiting values are specifically prescribed by the manufacturer and are used to ensure operation of the component in the so-called SOA range (SOA=Safe Operating Area).
If the component is switched off too rapidly, a so-called “latch-up” of the component may result. Latch-up is an operation in which a parasitic npn bipolar transistor which is formed from the n-doped source zone 15, the p-doped body zone 14 and the n-doped drift zone 12 switches on. As a result of this parasitic npn bipolar transistor switching on, a parasitic thyristor which is formed from the source zone 15, the body zone 14, the drift zone 12 and the emitter zone 11 fires, thus resulting in it no longer being possible to control the component and in possible destruction. The parasitic npn bipolar transistor switches on when the hole current flowing away from the drift zone 12 when the component is being switched off is so high that the voltage drop caused by this hole current under the source zones 15 in the body zone 14 is greater than the threshold voltage of the parasitic bipolar transistor.
Regions of the cell array in which the cell density is reduced in comparison with other regions of the cell array and in which there are thus fewer terminal contacts to the source electrode 18 for the purpose of removing the holes from the drift zone 12 are particularly critical as regards the “latch-up” behavior. In the component according to FIG. 1, the reference symbol 102 is used to represent a region having such a reduced cell density. In the example, it is a region in which, in addition to the cell array, there is a gate lead 22 which is used to connect the gate electrode 16 to a gate potential in a low-impedance manner. There are no transistor cells and, in particular, no terminals to the source electrode 18 under this gate lead 22 in the drift zone 12. When the component is being switched off, holes must flow away, under these gate leads 22, from the region of the drift zone 12 via the body zones 14 of transistor cells which are arranged adjacent to this region 102 with a reduced cell density. When the component is being switched off, the hole current density is thus increased in these adjacent transistor cells in comparison with other transistor cells in the cell array which are arranged at a greater distance from the region 102, with the result that the risk of a “latch-up” is particularly high as regards these transistor cells. In this case, these transistor cells limit the maximum permissible current or voltage changes for the entire component when switching off the component.
In this case, the current or voltage changes which occur in the component when it is being switched off are smaller the slower the switching-off operation. However, the switching losses increase as the duration of the switching-off operation increases.